British Radar, 1939

At the outbreak of war in September 1939, CH had eighteen stations covering the eastern and half of the southern coast of Britain reporting to one Filter Room. The choice of HF imposed steep practical limitations on the system. HF, which is a relatively long wavelength, requires large antenna arrays to radiate sufficient power. Transmission at any one station required four 360-foot-high masts, 180 feet apart, between which the antenna wires were strung. The returned signal was not received by the same antenna, but on four separate 240-foot-high masts. To say the least the whole installation was extremely large. It could not rotate and did not scan, but floodlit a 100-degree sector with radiation.

Detection of aircraft was possible only within the limits of the 100-degree sector and depended on the direction finding of the return signal from various antennas. The system was ineffective over land and was only suitable to a coastal location.

Chain Home Radar
[Crown Copyright]

The CH system was the only English radar system in operation at the outbreak of the war. CH was a dead end approach to radar technology, whereas Freya would become the classic model of modern radar design. Freya was a mobile 360 degree radar effective over land and water, able to transmit and receive from the same antenna and able to resolve the target with a high degree of precision. The British failed to develop gun laying radar such as Würzburg, for flak batteries and Seetakt for naval guns, in the pre-war years. Even more telling, the RAF did not anticipate the need for navigational and bombing aids until confronted with German systems and their own inability to destroy targets with any reliability.

As important as CH was to the defence of Britain, the true advantage for Fighter Command was in the Filter Room. The Filter Room played a key role. CH stations were not effective in resolving and locating targets as was a Freya type radar which rotated and used a frequency five times higher. The Filter Room helped to minimize the weaknesses of CH. It was able to collect, and resolve into a clear picture, what the actual threat was from the numerous overlapping radar plots reported from various stations and match fighter resources against the enemy. The British had developed a lead over the Germans in the method in which they used radar information, but not in the equipment itself. Radar was just a component of the air defence picture. Spotting reports and signals intelligence filled in the areas where radar could not see, aircraft over 120 miles away and behind the radar station. In many ways signals intelligence was just as valuable to the British as was CH radar. With signals intelligence the British were able to repeat their naval successes of World War I in the new field of air combat.

Signals intelligence allowed radar to report the approach of aircraft which were already expected. CH was able to give a twenty minute warning to the fighters to intercept their target, but the radar was not sensitive enough to resolve the number of aircraft or type. German air communications were intercepted at British HF listening stations. Early in the war German fighters used HF radio telephony while the bombers used more traditional HF telegraph for communications. From the interception of this traffic, the British could get up to a two hour warning and detailed information on aircraft numbers, routes and identity of attacking formations.

Probably the biggest failure of the British radar effort was their inability to believe that the Germans had radar and the view that their technology was superior. This refusal to speculate on the existence of German radar is curious, given the amount of intelligence available. Such evidence included a detailed scientific report spirited to London from Norway, the Oslo Report, which detailed German developments in radar and rockets. It was discounted as a ruse and only after these revolutionary technologies were discovered was the report re-examined. The British also had photo intelligence of the radar array on the scuttled pocket battleship Graf Spee and the empirical evidence that their bombers did not get through at Wilhelmshaven, combined with the ability of German searchlights, fighters and flak to find their aircraft at night.

It is tragic to see that the lessons learned by the scientific community to defeat the German aerial threat were not considered in developing future bomber tactics. The use of radar and efforts to defeat German navigation aids did not cause a rethinking of tactics inside Bomber Command. It was not until 1942, when confronted with the fact their bombs were missing their intended targets and the credibility of the German defence, did Bomber Command admit their need for navigation aids and a change in tactics.

Later during the conflict, the radar war reached new heights of sophistication. Here, the German antenna of Klein Heidelberg, a passive system which exploited the transmissions from the British Chain Home (CH) stations, captured in Normandy.
[US National Archives]

Deflating the Myths

The post-World War II accounts of air battles and achievements recorded in Allied memoirs and histories painted their efforts in the best light. Dr. Watson-Watt was hailed as the father of radar and his invention a key component in the defeat of the Luftwaffe over the English skies. The deep British feeling that they invented radar, and their wholehearted discounting of German ability to produce such equipment, led to grave failures in their bombing strategy.

Britain led the world in a political appreciation of the strategic importance of radar. Political forces, not military initiative, pushed the pursuit of a solution to the bomber threat posed from Germany. Britain selected a high risk course of a radar based defence with technology that was inferior to the German systems. The technology of radar was not the sole possession of Britain, in fact better examples of it were in use in Germany. However, their appreciation of its capabilities and role in air defence was unique in 1939. The idea that Germany only developed radar after discovery of the British systems is just not true. Dogmatic British denial of the existence of German radar contributed to this myth. Air Marshal Joubert, as late as 23 February 1941, was conducting meetings to discuss whether the Germans had radar at all.

General Ismay, the British Chief of Staff reinforced this feeling of invented first in Britain through his claim that the British Expeditionary Force had left radar secrets in France. Watson-Watt’s assertions that captured French electronic firms were responsible for revealing the radar secret to the Germans further supported this view. In reality the Germans thought the captured British equipment was crude and inferior to their own designs.

Paradoxically, it was a German signals officer who worried about the British having similar technology and launched several Zeppelin reconnaissance missions to ascertain the function of the CH towers. Only the German’s own theory of radar, which discounted HF as a workable medium, prevented them from discovering the mystery of the 300-foot towers on the coast.

In tales of the Battle of Britain radar emerges as the sword that defeated the Luftwaffe during the ‘Blitz’. Radar was just a key component in the system of command and control that Dowding and his staff at Fighter Command developed. Their innovation was to meld the potential of radar into an integrated system of rapid counter-action against bomber attack. The real hero was the unique development of the Filter Room which could sort all available intelligence and erect the best defence possible. Surprisingly, radar was not so unique, but the Filter Room at Bentley Priory was.

In 1939 Germany possessed some of the most advanced radar designs and yet failed to see its most powerful use as an early warning and fighter command and control device. The focus on supporting Blitzkrieg blinded the Luftwaffe leadership. The German navy developed and presented the Luftwaffe with a mobile search radar, Freya, effective over land and water. In contrast to the huge fixed Chain Home radar which was only effective over water. Used in conjunction with Freya, Würzburg was a high-precision targeting radar which could guide anti-aircraft artillery shells, searchlights and aircraft. These enduring radar designs provided the Germans with the ability to develop a more capable air defence to blunt an ever increasing air assault.

Not only were their radar designs superior, but the Germans had grasped the importance of radio navigation and bombing aids to support bombing offensives. They displayed night bombing capabilities which Bomber Command would not be able to match until 1942.

Even when deflating the World War II radar myths, it is important to realize that even though the British initially deployed a more primitive radar design, they had developed the correct strategy to defeat the German air campaign. Germany’s fixation with the offense had blinded them to the possible defensive uses of their own radar, let alone what it would do to their offensive strategy, if an enemy possessed it. Conversely, the British failed to apply the lessons learned about German bombing and navigation aids and the hazards of unescorted daylight bombing against areas defended with radar.